Fixed bed adsorption systems are well known. Such systems and their many applications are discussed, for example, by H. M. Barry, Chem. Eng., Feb. 8, 1960, pp. 105-120. Such systems are also discussed in a three part series by G. M. Lukchis, Chem. Eng., June 11, 1973, pp. 111-116, July 9, 1973, pp. 83-87 and Aug. 6, 1973, pp. 83-90.
Briefly adsorption is the condensation of gas (or liquid) molecules on the surface of a solid due to attractive forces, called Van der Waal's forces, which are similar in nature to those forces responsible for surface tension in liquid. Since the total surface energy of the adsorbent is equal to the product of the surface energy per unit area and the total surface area, large surface is the primary consideration in the selection of a material for use as an absorbent. The requirement of large surface area and the desire to minimize the volume of the adsorption system leads to the use of highly porous solids as adsorbents. Among the widely used adsorbents, as discussed in the foregoing references, is, for example, activated carbon.
The force of attraction of the adsorbent is not the same for all gas molecules. Rather, the force of attraction is related to the size of the pores of the adsorbent relative to the size of the adsorbed molecule. The more nearly the pore size is to the diameter of the adsorbed molecule, the greater the attraction. This phenomenon results in selective adsorption of various gases whereby the adsorption process can be used for the separation of such gases.
In known fixed bed adsorption gas separation systems the inlet gas mixture is applied to a bed of adsorption material until breakthrough of the adsorbate (the adsorbed gas) at which time the inlet gas is diverted to a parallel bed and the saturated bed is then regenerated with heat and/or vacuum. The degree of concentration of the adsorbate obtained during one such cycle is limited by the co-adsorption of other gases on the bed. Therefore, if a higher concentration of the adsorbate is required, the adsorbate is applied to a second stage bed, etc. To achieve high concentration in this manner, many stages are required.
An object of the present invention is to improve the concentrating efficiency of each stage to thereby reduce the amount of equipment and, therefore, the cost of a gas separation system.